This invention relates to filters, and in particular to filters constructed using bulk acoustic wave resonators. Such filters may be used in communications equipment as band pass filters which enable selection of a frequency band in which transmission channels are located, and with rejection of frequencies outside the band of interest. The invention also relates to communications equipment (for example, a radio frequency receiver and/or transmitter) comprising such filters.
High-performance radio-frequency (RF) filters typically use high dielectric constant ceramic resonators or surface acoustic wave resonators. The former devices are rather bulky, whereas the latter are smaller but have higher insertion loss (generally greater than 3 dB) and generally rather poor stop-bands. As a result, neither provides an ideal solution for channel band selection in small communications devices such as mobile phones. Filters for such applications need deep stop-bands to reject unwanted signals, as well as low pass-band insertion loss (typically less than 2 dB) to achieve adequate signal-to-noise ratio. There is therefore a requirement for very small resonators with high Q-factor (typically greater than 500). To achieve this aim, with potential for integration on silicon, thin-film bulk-acoustic-wave (BAW) resonators have been proposed. These are applicable to the frequency range 0.5 to 10 GHz, and are therefore appropriate for the third generation mobile telephony standard, as well as for already established wireless standards, such as GSM, W-CDMA, Bluetooth, HomeRF, DECT and GPS.
The need for low insertion loss and high stop-band attenuation can not be achieved with a single resonator. Filters are therefore typically made up of a number of resonators, and a conventional thin-film BAW filter configuration is a ladder construction, shown in simplified schematic form in FIG. 1. This has alternating series sections 2 and shunt sections 4, each of which can be a single resonator, or one or more resonators on the same frequency connected in series or parallel (which is electrically equivalent). The anti-resonant frequency of the shunt element is chosen to be the resonant frequency of the series elements to provide minimum insertion loss at that frequency.
The individual resonators are typically arranged as so-called solidly-mounted resonators (SMRs), an example of which is illustrated in FIG. 2. The required conversion between electrical and mechanical energy is achieved by a layer of piezoelectric material 10 (for example zinc oxide, aluminium nitride, PZT, PLZT) between two metal layers 12, 14 in which electrodes are formed. The piezoelectric material 10 is provided over one or more acoustically mismatched layers 16, which are mounted on an insulating substrate 18, for example glass. The acoustically mismatched layers act to reflect the acoustic wave which results from resonance of the piezoelectric layer 10 at the resonant frequency.
In FIG. 2, a number of high impedance layers 16a and low impedance layers 16b are shown. Porous silicon oxide (aerogel) may be used for the low-impedance 16b layers, and a single layer may in fact be adequate to achieve sufficiently high Q, due to the very low acoustic impedance of this material. The high impedance layers 16a may comprise tungsten.
In FIG. 2, the upper metal layer 12 defines both terminals 12a, 12b of the resonator, and the lower metal layer 14 effectively acts as an intermediate electrode between two series-connected resonators. This avoids the need to make electrical contact to the lower metal layer 14 through the piezoelectric layer 10. A single pair of series-connected resonators then acts as the basic building block of the filter and may be considered as the basic resonator element. FIG. 2 also shows a plan view, with contact pads 20 providing the input and output of the device.
Ladder filter arrangements such as shown in FIG. 1 have demonstrated good performance, for example less than 2 dB insertion loss and very low-level of spurious response. However, there are also some disadvantages, which can be understood from an approximate electrical equivalent circuit of the resonator, shown in FIG. 3.
Co is an (unwanted) static capacitance of the resonator, whereas Cm, Lm and Rm characterise the mechanical resonance. These are, respectively, the motional capacitance, motional inductance and motional resistance of the resonator. The resonator appears as a pure capacitor Co at frequencies removed from the resonance (except at other significant mechanical resonances such as harmonics, which are not accounted for in this simple model). In designs reported to date, the shunt and series resonators have similar areas, and therefore similar static capacitances. This gives only about 6 dB attenuation, in the frequency bands to be rejected by the filter (the xe2x80x9cstop-bandxe2x80x9d), per combination of series and shunt sections. This is the result of the static capacitance of each resonator. A T-section, comprising two series-connected resonators and an intermediate shunt resonator may can be considered as the basic building block of a ladder filter. A single resonator element 2i, 2o (FIG. 1) is then at the input 6 and output 8 of the filter, and the intermediate series resonators elements 2b each comprise two series-connected resonator elements.
To achieve the desired low pass-band insertion loss and high stop-band insertion loss, each individual building block should meet these two requirements. Although increasing the number of sections adds to the stopband loss (as required), this also increases pass-band loss (and also the overall filter size). The pass-band and stop-band requirements therefore conflict with each other. Typically, several such building blocks are required for even moderate stop-band rejection. Consequently, both the area occupied and the insertion loss in the pass-band are increased without improving filter selectivity.
It has been recognised, for example in U.S. Pat. No. 5,471,178, that the stop band performance for a ladder filter is determined in part by the static capacitance ratio between the series and shunt resonators, as the resonators act as a capacitive voltage divider at frequencies removed from the resonant frequencies.
According to the invention, there is provided a ladder filter comprising a plurality of bulk acoustic wave resonators, the resonators comprising a plurality of series resonators in series between an input port and an output port of the filter, and one or more shunt resonators each connected between a junction between two series resonators and a common terminal, the series resonators comprising an input series resonator connected to the input port and an output series resonator connected to the output port, and wherein the or each shunt resonator has a static capacitance which is more than four times the static capacitance of the input or output series resonators.
The ladder filter of the invention provides increased shunt resonator capacitance (compared to conventional designs in which the series and shunt resonators have substantially the same area). This reduces the effective coupling across the section thereby enabling a smaller number of series-shunt filter sections to be used to achieve good stop-band rejection, while still providing good performance in the pass-band. The filter of the invention can be impedance matched to both input and output impedances of the filter, so as to minimise the pass-band insertion loss. The increased shunt capacitance does, however, reduce the filter bandwidth, and the invention is based on the recognition that filter bandwidth can be traded for improved out-of-band rejection.
The series resonators may further comprise one or more intermediate series resonators having a static capacitance which is approximately half the static capacitance of the input or output series resonators. In this way, the ladder filter can be made up of identical T-section building blocks. For equal input and output impedances, the input and output series resonators preferably have the same static capacitance.
Each bulk acoustic wave resonator preferably comprises a layer of piezoelectric material sandwiched between two metal layers in which electrodes are formed, the piezoelectric material being provided over a plurality of acoustically mismatched layers mounted on an insulating substrate. The piezoelectric material may comprise PZT.
The ladder filter may be used to define a band pass filter having a centre frequency in the band pass region, wherein the series and shunt resonators are tuned such that the series resonators have an impedance minimum at the centre frequency and the shunt resonators have an impedance maximum at the centre frequency. This provides maximum coupling between the input and the output at the resonant frequency.
The series and shunt resonators may be designed to satisfy:
Cseries=(2xcfx89shunt)/((xcfx89series2Rom)
Cshunt=(2xcfx89shuntm)/((xcfx89series2Ro)
where Cseries is the static capacitance of the input and output series resonator,
Cshunt is the static capacitance of the or each shunt resonator,
xcfx89series and xcfx89shunt are the angular resonant frequencies respectively of the series and shunt resonators,
Ro is a desired input and output impedance of the filter, and
m is a parameter which is equal to the square root of the ratio of shunt-to-series static capacitance.
This arrangement enables the filter to be matched to a desired input and output impedance to achieve low pass band loss. The greater the value of m, the greater rejection in the stop-band achieved by each resonator stage. Selection of the ratio m dictates the capacitance of the series and shunt resonators, and the areas of these components over the substrate can then be selected accordingly. There is, however, a practical upper bound on m which depends on the coupling factor K of the piezo-electric film, resonator quality factor Q, and the required band-width. The value of m is greater than or equal to 2 according to the invention, and may take a value between 2 and 32.
The invention also provides a radio frequency band pass filter comprising a ladder filter of the invention. A radio frequency receiver and/or transmitter may use such a band pass filter.